This invention relates generally to phase shift modulators and more particularly to digital binary phase shift key (BPSK) modulators.
As is known in the art, phase modulation, sometimes referred to as phase shift keying, is one technique available for communicating data over information channels. In some applications, phase shift keying has the advantage of requiring less peak power than multi-level amplitude modulation and may have a reduced bandwidth requirement when compared to frequency modulation. Phase modulation techniques have been successfully employed to transmit digital data over high-frequency channels where fading and multipath phenomena are often encountered. In these applications, information is conveyed in the form of phase differences between successive radio frequency pulses.
The simplest technique of phase modulation is called binary phase shift keying for modulating between two states where the phase difference between the two states is generally 180.degree.. Accordingly, a 180.degree. phase shifter is most often used as a BPSK modulator.
There are many types of phase shifters including switched line, reflection, loaded line, and high pass/low pass phase shifters.
A switched-line phase shifter generally makes use of a pair of single pole double throw (SPDT) switches for switching between two separate transmission line paths. The first path has a transmission line length l, while the second path has a transmission line length l+.DELTA.l for providing a phase delay longer than the first path given by EQU .DELTA..phi.=2.pi.l/.lambda.
It is important to note that the phase shift varies as a function of wavelength and accordingly is not constant over frequency.
Another type of phase shifter used in many modulator applications is the reflection phase shifter which includes a switching diode used to switch between a pair of reflective loads for providing signals having a difference in phase shift. A duplexer circuit, often a ferrite circulator, is used to provide the input signal and for extracting the phase shifted output signal. The diode, usually a PIN diode, switches a signal between two low loss impedances to provide signals having a differential phase shift.
The high pass/low pass phase shifter is often used in applications requiring broadband performance and compact size. In the high pass/low pass phase shifter configuration, the low pass filter section includes series inductors and shunt capacitors for providing phase delay to signals passing therethrough. Conversely, the high pass filter is comprised of series capacitors and shunt inductors for providing phase advance to the signals. The switch elements permit switching between the low pass and high pass sections, to provide a compact phase shifter having broader band performance than the previously mentioned reflection, switched-line or loaded line type phase shifters.
The insertion loss and differential phase shift errors of the above described phase shifter configurations limit the bandwidth over which these phase shifters may be used. Although, monolithic microwave integrated circuit implementations generally provide better insertion loss and phase shift error characteristics than hybrid circuit arrangements and lower variations in electrical characteristics from unit to unit, the errors still limit the operating bandwidth of these circuits.
For example, the above described high pass/low pass phase shifters may be provided as a monolithic microwave integrated circuit (MMIC) since the switch elements are typically fabricated from FET devices such as MESFETs. Typical small-sized MMIC high/low pass type phase shifting networks having nominal 180.degree. bit phase shift have insertion loss errors as high as .+-.1 dB and differential phase shift errors of 180.degree..+-.25.degree. over the 6-18 GHz frequency range. In applications requiring multi-octave bandwidths these insertion loss and phase shift errors are still considered to be too large for many communication and electronic countermeasure (ECM) systems.